An ion generating apparatus is commonly employed for implantation of ions on a silicon wafer during semiconductor device fabrication. The primary components of an ion generating apparatus include an ion generator 4, an ion selector/deflector 6, and an ion accelerator 8, as illustrated in Prior Art FIG. 1. Control over ion energy levels, reduction of ion implant time, and elimination of ion impurities are primary considerations of such a process.
The ion generator 4 includes a dual head for generating ions, a power supply for supplying power to the dual head to generate thermal electrons, an ion gas source which releases ions when energized by the thermal electrons, and other related components. The amount of ions produced by the ion generator 4 is a function of several variables, including the volume of source gas flow, the degree of thermal electron emission, and the efficiency of the interaction therebetween for ionizing the source gas.
The ion selector/deflector 6 selects ions from those generated by the ion generator 4 and deflects them toward a reaction chamber in which a wafer is loaded. In general, the selection process and deflection process occur contemporaneously.
The ion accelerator 8 propels the selected/deflected ions into the wafer. The level of ion acceleration is determined by the degree of energy required to implant ions to the wafer. The accelerated ions are implanted over an entire surface, or alternatively, a predetermined region, of the wafer.
FIG. 2 is a schematic illustration of an ion generator 4 including a conventional ion generating means referred to in the art as a dual head. The dual head comprises a reaction chamber 10 for generating ions, and electromagnets 20a, 20b installed on opposite sides of the reaction chamber 10. A common power supply P4 is connected to coils 21a, 21b winding the electromagnets 20a, 20b. The electromagnets 20a, 20b induce a magnetic field 24 having a predetermined intensity inside the reaction chamber 10.
The reaction chamber 10 is an arc chamber, and thus an arc voltage P1 is applied thereto. The reaction chamber 10 includes filaments 12a, 12b to which external power supplies P2 and P3 are connected. The filaments 12a, 12b emit thermal electrons 22 which provide the basis for generating ions. The applied external power levels P2 and P3 control the emission of thermal electrons 22. Floating repellers 14a, 14b are installed on the opposing inner walls of the reaction chamber 10. The repellers 14a, 14b pass through the walls of the reaction chamber 10 through insulating bodies 16a, 16b, and guide ions generated in the reaction chamber 10 toward aperture 18 for emission therefrom. The upper ends 13a, 13b of the filaments 12a, 12b are disposed between the repellers 14a, 14b. The reaction chamber 10 is an enclosed chamber with the exception of an ion emission aperture or hole 18 formed in the upper part of the reaction chamber, facing the upper ends of the filaments 12a, 12b.
When a voltage is applied to the filaments 12a, 12b, thermal electrons 22 are emitted from the upper ends 13a, 13b. Thermal electron 22 emissions may be increased or decreased by controlling the applied voltages P2, P3, as described above. Thermal electrons 22 collide with ion generation source gases (not shown) introduced into the reaction chamber 10, whereby the source gases are ionized, forming free ions in the reaction chamber 10. The free ions are guided to the center of the reaction chamber 10 by the repellers 14a, 14b and exit the reaction chamber 10 through the emission hole 18. The emitted ions 19 are implanted into a wafer via the ion selector/deflector 6 and ion accelerator 8 (see FIG. 1).
The ionization rate of the source gases in the reaction chamber 10 can be increased by raising the applied voltage levels P2, P3 thereby heightening emission activity of thermal electrons. However, this results in increased energy consumption and is generally inefficient. In a more effective technique, the reaction chamber 10 is interposed in a magnetic field 24 generated by electromagnets 20a, 20b. As a result, when thermal electrons are emitted, they propagate along a spiral path 23 in the magnetic field 24 according to electromagnetism theory. The spiral motion 23 increases the efficiency of ion emission in the reaction chamber by heightening the number of collisions between the thermal electrons 22 and the source gases. However, the increase in efficiency comes at a cost, as the thermal electrons 22 tend to spiral toward one of the electromagnets 20a, 20b. For example, the thermal electrons 22 are urged toward to the south (S) pole of the electromagnets 20a, 20b, as electromagnetic forces generated by the electromagnets 20a, 20b proceed from the north (N) pole to the S pole. As electrons 21 collect at the S pole, the potential energy of the filament 12a near the S pole electromagnets 20a increases and thus the filament 12a near the S pole electromagnet 20a emits more thermal electrons than the filament 12b near the N pole electromagnet 20b. As a result, the repeller 14a near the S pole collides with many thermal electrons 25a, and a great number of collided thermal electrons 25b collide with the filament 12a near the S pole. Accordingly, the durability of the filament 12a is reduced, and the maintenance or replacement cycle of the ion generation parts is shortened.